This invention relates to a flexible architecture of a telecommunications system using datagrams, such as ATM.
The volume of voice and voice-band calls is increasing markedly, and network providers are being challenged to offer these "plain old telephone" services at competitive prices. ATM presents an opportunity to reduce costs, and is therefore being considered for carrying circuit-switched voice traffic. Conventionally, a circuit-switched network is managed by formulating a logical view of the network that includes a link between most pairs of network switches, and the network is managed at the logical level. The logical view does not necessarily correspond to the actual, physical, network. The logical connections over which routing is performed ride on a facility network. The facility level contains the physical switches and transmission resources. The connections demanded at the logical level are mapped into demands on the facility network. Routes that appear as direct at the logical level may pass through many cross-connects at the physical level.
The partitioning of a circuit-switched network into logical and physical layers results in significant inefficiencies. Physical diversity is difficult to plan for such networks due to the indirect mapping between the logical and physical layers, and such networks have high operations costs due to the constant need to resize trunk groups between switch pairs as the load changes or shifts. Also, sharing of bandwidth is limited to the possible alternate paths at the logical layer. Finally, such networks are difficult to scale as network traffic increases because each switch that is added to the network must be interconnected to all other switches at the logical layer, trunks on existing switches must be re-homed to the new switch, and the routing tables at all other switches in the network must be updated. All of this creates substantial operational load on the network provider. Since facilities are in units of T3 capacity, fragmentation of trunk groups also increases with the size of the network.
ATM networks have the potential to eliminate some of the inefficiencies in traditional circuit-switched networks. In an ATM implementation that creates circuit connections, the logical and physical network separation may or may not be maintained. Voice calls in such a network may be treated as ATM virtual circuits, which may be either Constant Bit Rate (CBR) or Variable Bit Rate (VBR) arrangements, depending on the voice coding scheme. These virtual circuits may be set up using standardized ATM setup procedures and routing protocols--as, for example, in the Private Network-to-Network Interface (PNNI) specification. However, the standard procedures of an ATM network require the ATM switches in the network to perform a substantial amount of computations, which is burdensome and which makes it difficult to operate the network at high load capacities.